All U.S. patents cited below are herein entirely incorporated by reference.
As used herein, the term “thermoplastic” is intended to mean a polymeric material that will melt upon exposure to sufficient heat but will retain its solidified state, but not prior shape without use of a mold or like article, upon sufficient cooling. Specifically, as well, such a term is intended solely to encompass polymers meeting such a broad definition that also exhibit either crystalline or semi-crystalline morphology upon cooling after melt-formation through the use of the aforementioned mold or like article. Particular types of polymers contemplated within such a definition include, without limitation, polyolefins (such as polyethylene, polypropylene, polybutylene, and any combination thereof), polyamides (such as nylon), polyurethanes, polyester (such as polyethylene terephthalate), and the like (as well as any combinations thereof).
Thermoplastics have been utilized in a variety of end-use applications, including storage containers, medical devices, food packages, plastic tubes and pipes, shelving units, and the like. Such base compositions, however, must exhibit certain physical characteristics in order to permit widespread use. Specifically within polyolefins, for example, uniformity in arrangement of crystals upon crystallization is a necessity to provide an effective, durable, and versatile polyolefin article. In order to achieve such desirable physical properties, it has been known that certain compounds and compositions provide nucleation sites for polyolefin crystal growth during molding or fabrication. Generally, compositions containing such nucleating compounds crystallize at a much faster rate than un-nucleated polyolefin. Such crystallization at higher temperatures results in reduced fabrication cycle times and a variety of improvements in physical properties, such as, as one example, stiffness.
Such compounds and compositions that provide faster and or higher polymer crystallization temperatures are thus popularly known as nucleators. Such compounds are, as their name suggests, utilized to provide nucleation sites for crystal growth during cooling of a thermoplastic molten formulation. Generally, the presence of such nucleation sites results in a larger number of smaller crystals. As a result of the smaller crystals formed therein, clarification of the target thermoplastic may also be achieved, although excellent clarity is not always a result. The smaller crystal size, the less light is scattered. In such a manner, the clarity of the thermoplastic article itself can be improved. Thus, thermoplastic nucleator compounds are very important to the thermoplastic industry in order to provide enhanced clarity, physical properties and/or faster processing.
The most effective thermoplastic nucleator in terms of high crystallization temperatures is available from Milliken & Company under the tradename of HPN-68. Other like thermoplastic nucleating compounds are disclosed within U.S. Pat. Nos. 6,465,551 and 6,534,574, both entirely incorporated herein by reference. The HPN-68 compound is disodium bicyclo[2.2.1]heptanedicarboxylate. Other thermoplastic nucleating agents that exhibit appreciably lower crystallization temperatures include dibenzylidene sorbitol compounds, such as 1,3-O-2,4-bis(3,4-dimethylbenzylidene) sorbitol (hereinafter DMDBS), available from Milliken & Company under the trade name Millad® 3988, sodium benzoate, sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl) phosphate (from Asahi Denka Kogyo K. K., known as NA-11), talc, cyclic bis-phenol phosphates (such as NA-21, also available from Asahi Denka), and, as taught within Patent Cooperation Treaty Application WO 98/29494, to Minnesota Mining and Manufacturing, the unsaturated compound of disodium bicyclo[2.2.1]heptene dicarboxylate. Such compounds all impart relatively high polyolefin crystallization temperatures; however, each also exhibits its own drawback for large-scale industrial applications, and none can match the effectiveness of the above-noted saturated types.
Some of the above-noted nucleating agents also provide clarifying properties within certain thermoplastics, such as polypropylene (Millad® 3988, for example, and to a lesser extent, NA-21). Such clarification capabilities coupled with high peak crystallization temperatures are highly desired. For certain end-uses, at least a maximum level of haze (for instance, 35%) is acceptable. The previously listed dicarboxylate salt nucleating agents unfortunately exhibit relatively high haze levels within polypropylene, although such compounds also provide excellent calcium stearate compatibility, increased stiffness within target thermoplastic articles, and certain degrees of hygroscopicity. Thus, such compounds provide extremely desirable qualities and benefits within target thermoplastics. Unfortunately, as noted above, haze problems have limited the usefulness of such nucleating agents within certain target end-uses, even though the crystallization temperatures imparted thereby are extremely high.
To remedy this initial problem, haze reduction has been achieved when such saturated dicarboxylate salts have been either spray dried to form relatively large particles, or jet-milled for substantially uniform small particle sizes (from 2.5-4.5 micrometers in length). However, it has unfortunately been noticed that upon producing such small particle size compounds, there is a tendency for the compounds to suffer from stacking and eventual agglomeration (which inevitably leads to cementation of the stored solid compounds), thus deleteriously affecting the ability to actually disperse, if not use altogether, such compounds in thermoplastic media. Additionally, during storage such compounds exhibit “growth” due to such agglomeration as well within the packaging container such that it has been noticed on regular occasions that the storage container itself becomes ruptured and/or damaged and the nucleator powders leak therefrom or it becomes very difficult to remove the cemented product therefrom. These problems are most likely due to the plate-like structures such compounds exhibit coupled with exposure to moisture and/or humidity. Unlike cubic, spherical, or other like geometric shapes, such plate-like configurations are highly susceptible to the aforementioned stacking problem. When such occurs, particularly in air jet-milled or spray dried, substantially uniform small particle-size samples, it has been realized that even a small amount of moisture can lead to molecular attraction between two plate-like structures thereof. Upon bonding, the ability to separate such structures is extremely difficult. Upon stacking of a larger number of such structures, cementation and “growth” (increase in volume within a closed space) may occur, thereby preventing use thereof of the particular sample and/or resulting in difficulties with storage within tightly sealed containers. Furthermore, it has been found in some circumstances that such cemented samples are bonded to such a degree that separation is, for the most part, impossible. Unless such small particles can actually be added and dispersed within target prepolymer media, the benefits of nucleation and possible clarification are simply unavailable. Thus, this cementation problem prevents effective utilization of such an excellent thermoplastic nucleating agent, especially for purposes of imparting lower haze levels.
As such, there is a definite need to prevent plate-to-plate interactions of individual saturated dicarboxylate salt thermoplastic nucleating agents, particularly during production, storage, and incorporation within target thermoplastic media. In such a manner, it is theorized that the substantially uniform small particle-sized compounds could then impart the desired lower haze levels than for the larger and/or nonuniform particle size compound formulations. Without such needed remedies, the ability to utilize such an extremely effective and efficacious thermoplastic nucleating agent is limited to opaque end-uses.